1. Field of the Invention
The present invention relates to electronic read-out devices, and, more particularly, to devices for providing improved electronic read-out of magnetic measurements made by direct current superconducting quantum interference devices (DC SQUIDs).
2. Description of the Prior Art
DC SQUIDs are small, cryogenically-cooled magnetic sensors that comprise a ring of superconducting material interrupted by two Josephson junctions. DC SQUIDs are designed to detect changes in magnetic flux, and, when suitably biased with a small DC current, will exhibit a magnetic flux sensitivity noise floor of approximately 1xc3x9710xe2x88x926 "PHgr"0/{square root over ( )}Hz for low temperature devices that operate near 4 degrees Kelvin (typically cooled by liquid Helium), and approximately 7xc3x9710xe2x88x926 "PHgr"0/{square root over ( )}Hz for high temperature devices that operate near 77 degrees Kelvin (typically cooled by liquid Nitrogen). Furthermore, DC SQUIDs exhibit a transfer function that converts magnetic flux into a periodic electrical output signal.
The standard read-out method for DC SQUID measurements is to inject an alternating current (AC) magnetic field modulation signal into the DC SQUID and then, using a flux locked loop (FLL) circuit, sense changes in the modulating signal due to external magnetic fields. The FLL maintains a stable magnetic flux operating point at the DC SQUID by introducing a feedback magnetic flux that precisely counteracts the externally applied magnetic field, provided the slew rate and dynamic range of the DC SQUID and FLL are not exceeded. Measurements of the external magnetic flux can be made by measuring the feedback signal which is an identical image of the external magnetic flux signal within the tracking bandwidth of the FLL.
DC SQUID sensor systems for non-destructive testing/evaluation of materials or structures or for biomagnetic measurements are not yet practical for use in a field setting (i.e., environments containing high levels of magnetic interference). The art has been limited to a flux modulation frequency of approximately 500 KHz with a maximum tracking loop bandwidth of 250 KHz. In magnetically unshielded environments, large amplitude or high slew rate external stray magnetic fields from 50/60 Hz AC power lines, AM broadcast transmitters, small changes in the Earth""s magnetic field, and other sources, cause the FLL to lose lock, thereby invalidating any measurement in progress. Furthermore, the prior art employs traditional twisted-pair wires which are highly undesirable for several reasons: they have a high degree of linear attenuation versus frequency that severely distorts square waves of even moderate frequencies, they allow for a large amount of radiated leakage and corresponding susceptibility to radio-frequency interference, and they have a highly variable characteristic impedance that changes with mechanical stress and is difficult to impedance match.
The incorporation of digital signal processing (DSP) technology into the FLL of a DC SQUID has been attempted, but results have been limited due to inherent delays associated with signal acquisition, processing and reconstruction of the feedback signal, and the maximum clock frequency of the DSP. Because of these problems, previous attempts to incorporate DSP into an FLL have failed to increase the operating frequency above that obtainable with standard analog read-out systems.
For these reasons, DC SQUIDs are restricted to controlled environments which are shielded from magnetic interference and are typically expensive, bulky, and non-portable.
The read-out electronics of the present invention include essential enabling technology which makes the operation of DC SQUIDs practical in unshielded environments by alleviating the effects of high levels of magnetic interference on DC SQUID measurements. More particularly, the read-out electronics of the present invention incorporate innovative circuit designs that extend the frequency of operation of FLLs and improve upon the prior art by a factor of at least ten. Furthermore, the present invention employs DSP algorithms to filter, extract, and measure the desired weak signal. The problems encountered in prior attempts to incorporate DSP technology into DC SQUID read-out electronics have been overcome in the present invention by locating the DSP outside of the FLL.
In the read-out electronics of the present invention, traditional twisted-pair wires are replaced by shielded, unbalanced, controlled-impedance transmission lines which overcome many of the problems encountered in the prior art, including reducing the amount of radiated leakage and corresponding susceptibility to radio-frequency interference.
The primary application of the present invention is in magnetic measurement and non-destructive testing/evaluation (NDT/NDE) of materials and structures. There are both weapon and non-weapon NDT/NDE applications; the most notable non-weapon application being the detection of hidden cracks and corrosion in aging aircraft structures. The invention may also have application in making biomagnetic and geomagnetic measurements.
These and other important aspects of the present invention are more fully described in the section entitled Detailed Description, below.